Is it possible to use an iphone with a s4 sim card?

[hihi111]

I currently have a Samsung S4 on a good plan. I am wondering if I could just switch out my SIM into the new iphone 5s without having to go into an expensive MAX contract. I would be buying an unlocked phone from apple. 

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[sheedy17]

Regardless what kinda SIM card you use, you can keep your plan if you buy an unlocked phone.